In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there have been, and continue to be, efforts toward scaling down device dimensions (e.g., at sub-micron levels) on semiconductor wafers. In order to accomplish such high device packing densities, smaller and smaller feature sizes are required. This includes the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry, such as corners and edges, of various features. The dimensions of and between such small features can be referred to as critical dimensions. With an ever increasing number of integrated circuit features being formed on a circuit die, the importance of properly designing patterns to form structures that are isolated and non-interfering with one another has also increased.
The requirement of small features with close spacing between adjacent features requires high resolution lithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon structure is coated uniformly with a radiation-sensitive film (the resist or lithographic coating) and an exposing source (such as optical light, x-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template. The intervening master template is generally known as a mask, photomask, or reticle for a particular pattern. The lithographic coating is generally a radiation-sensitive coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative image or a positive image of the subject pattern. Exposure of the coating through a reticle, mask or photomask causes the image area to become either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
The process of manufacturing semiconductors (e.g., integrated circuits, ICs, chips) employing masks typically consists of more than a hundred steps, during which hundreds of copies of an integrated circuit may be formed on a single wafer. Generally, the process involves creating several patterned layers on and into the substrate that ultimately fowls the complete integrated circuit. The patterned layers are created, in part, by the light that passes through the masks. A series of lenses provides for reduction in size from the mask to the projected image onto the resist. The optical equipment for traditional photolithographic processes requires significant capital investment.
Imprint lithography (also know as nanoprint lithography, nanoprint, nanoimprint or nanoimprint lithography) technologies are emerging which provide an alternative in which the capital investment is significantly reduced in part because patterns are exposed through a 1:1 mask or mold in close proximity to the wafer. Imprint lithography is relatively inexpensive because it avoids costly optics, as well as cumbersome enhancement techniques like phase-shift masks. Capital cost for equipment is far less than typical step-and-scan or scan and repeat systems. Imprint lithography does not depend on costly optical elements; rather, the line width is determined by the mask or mold.
One advantage of imprint lithography is that the circuit designers do not need to be concerned about optical proximity correction which limits how patterns are placed on the mask. Furthermore, patterning on top of a grating or other surfaces with severe topological features is possible providing significant advantages in MEMS applications.
Because imprint lithographic methods do not utilize the typical 4× optical reduction employed in conventional lithographic processes, however, the small feature sizes are more difficult to achieve. In order to produce devices with similar critical dimensions to conventional optical lithographic methods, new processes and techniques are required. Techniques which reduce the number of steps required to produce three dimensional features can further extend the economic advantages of imprint lithography.